In ethylene recovery and purification processes, several methods of refrigeration have been used or proposed in the art. Among these are cascade refrigeration processes, mixed refrigerant processes and expander processes.
Cascade refrigeration systems, usually employing ethylene and propylene refrigerants, are generally used in conventional large scale ethylene plants to recover ethylene from cracked gas. The cascade system provides refrigeration down to about -150.degree. F. Lower level refrigeration is supplied by J-T expansion of methane condensed from the feed gas, sometimes augmented by turbo expanders. Ethane separated from the product ethylene is recycled to the cracking furnaces for conversion to ethylene. Such processes are described in an article by W. Huang in the October 1980 issue of Hydrocarbon Processing, page 105-108, and in an article by V. Kaiser, et al, in the June 1979 issue of Hydrocarbon Processing, page 115-121, and in U.S. Pat. Nos. 4,496,381; 4,002,042 and 3,675,435.
Mixed refrigerant systems have been proposed to replace the ethylene-propylene cascade systems in conventional ethylene plants. Lower level refrigeration is again supplied by methane and/or turbo expanders. Again, separated ethane is recycled to the cracking furnaces. Such processes are described in an article by V. Kaiser, et al, in the July 1978 issue of Hydrocarbon Processing, page 163-167 and in U.S. Pat. No. 4,072,485.
Ethylene recovery processes which rely primarily on expander refrigeration have also been proposed, particularly for recovery of ethylene from refinery gas streams. One such process is described in an article by R. L. Rowell, in the May 10, 1982 issue of Oil & Gas Journal, page 127-131. This process is most suitable for low ethylene recovery, e.g. 40 to 80%. This process is also described in U.S. Pat. Nos. 4,272,270 and 4,356,014. The examples given in the patents are specific to C3+ recovery but the claims relate to C2 recovery also. A similar process is described by W. R. Minton in the Oct. 22, 1979 issue of Oil & Gas Journal, page 62-63. This process is claimed to be capable of achieving 85% ethylene recovery.
U.S. Pat. Nos. 4,035,167 and 4,401,450 describe processes using solvents, such as hexane or cyclohexane, to recover ethylene from gas mixtures. The latter patent also describes a regenerator process for ethylene recovery. Purification of the recovered ethylene enriched mixture is not discussed in either patent.